I think that every sitcom from the last 30 years containing one or more high school kids has done a "let's throw a party while your parents are out of town" episode. They all follow pretty much the same recipe: they party gets out of hand, stuff gets broken, the kids try to hide it from the parents. They usually get caught and punished, or they have to go to extremes in order to atone for their sins, but in the end they learn a valuable lesson and relationships are stronger as a result. They're formulaic, but the good ones can still find a way to be fun.
Project X starts off with the same old cliche. It's Thomas' 17th birthday, which just happens to coincide with his parents' anniversary. So they're going out of town to celebrate their anniversary, and they've given Thomas permission to have maybe four or five friends over for his birthday. Thomas is definitely not one of the cool kids at school, and in fact he's mostly invisible, so he lets his best friends Costa and J.B. talk him into a small party of say fifty people. But of Costa wants to do his friend a favor, so he invites a few more people, and they invite a few more people, and so on. And things start to get out of control.
However, there are some deviations from the old standard. This is a hard-R movie rather than an after-school special, so it's got a lot more swearing and nudity and sex crimes than you're used to from this kind of story, the destruction is more than just a broken vase or stained carpet, and there are substantially fewer lessons learned by the end. It tries to be Superbad, but fails miserably and just ends up being more crude and less funny. Actually, not at all funny. The only attempts at comedy in the film are via shock value, but the initial attempts are so blatant and ineffective that you're completely numbed to what comes later. I have no trouble at all believing that this was the work of a first-time (and hopefully only-time) director, but I'm shocked that Michael Bacall, who had a hand in the absolutely awesome Scott Pilgrim vs. the World and reportedly hilarious 21 Jump Street, could have also been responsible for this mess.
To further its failure, the film is told via found footage, and it's one of the sloppiest examples yet. With only a couple of exceptions, there's only a single camera operator (who's introduced with a derogatory slur and then is mostly ignored until a lame attempt at a joke near the end), and yet we see lots of different camera angles, in and out of the water, simultaneously up on the roof and down on the ground, etc. There's no reason they couldn't have made it like a traditional movie, but the characters' acknowledgement of the camera and its operator just creates a new way for this movie to fail.
It's hard to see anything of any value in Project X. It's basically the standard TV plot except that they've chopped off any hint of a moral and kept stuffing in crudeness until it was long enough to be considered a movie. If you're thinking about watching this, then just go with Superbad or Sixteen Candles or that episode of Saved by the Bell where they broke Screech's mom's Elvis statue, or any of the millions of other better examples of the same story.